1. Field of the Invention
The present invention relates to a form dampening roller driving apparatus that is applied to a form dampening device in an offset printing press.
2. Description of the Prior Art
FIG. 3 shows a conventional roller arrangement for the offset printing press having a plate cylinder 1 rotationally driven by a main motor (not shown). A dampening device 2 is provided for supplying the peripheral surface of the plate cylinder 1 with water, thereby dampening a form (offset) plate wrapped around it. The dampening device 2 includes a water fountain roller 4 that is rotated while being in part dipped in a water tank 3, a metering roller (chrome roller) 5 that is rotated in contact with the water fountain roller 4 so that the mutually contacting surfaces of the rollers 4 and 5 move in a same direction, a form dampening roller 6 that is positioned between the metering roller 5 and the plate cylinder 1 for supplying water adhered on the metering roller 5 onto the surface of the plate cylinder 1, and a vibrator roller 7 that is rotated in contact with the form dampening roller 6 so that the mutually contacting surfaces of the rollers 6 and 7 move in a same direction. Besides, the peripheral surface of the plate cylinder 1, namely the form plate carried thereon, is supplied with ink by an inking device 8 that comprises a plurality of (two as shown) inking rollers 9, 9 each in contact with the plate cylinder 1, an ink fountain roller 10, a pickup roller 11, and a distributing roller 12 juxtaposed with the ink fountain roller 10 via the pickup roller 11, and further a first group of rollers 8a for supplying ink on the distributing roller 12 onto one of the inking rollers 9 via the vibrator roller 7 in the form dampening device 2 and a second group of rollers 8b for supplying ink on the distributing roller 12 onto the other inking roller 9 via another vibrator roller 13, the inking rollers 9 applying ink so supplied to the plate cylinder 1.
The water fountain roller 4 and the metering roller 5 are geared to each other and rotationally driven by a motor that is independent of the main motor which rotates the plate cylinder 1. Also, the form damping roller 6 is held in pressure contact with the vibrator roller 7 and under the force of friction then applied thereto is designed to be rotated at a peripheral speed that is identical to that at which the vibrator roller 7 is rotated.
Driven to rotate by the main motor provided at the press machine body, the plate cylinder 1 and the vibrator roller 7 are rotated at an identical peripheral speed, and so is the form damping roller 6. Hence, the plate cylinder 1 and the form dampening roller 6 are allowed to rotate without slipping in their region of mutual contact S1.
Here, making a difference in peripheral speed between the metering roller 5 and the form dampening roller 6 causes a slip to occur between these rollers in their mutual area of contact S2, and it is possible to control the rate of water supply onto the form dampening roller 6 as a function of the degree of this slip. Accordingly, the arrangement is designed to change the rate of water supply by changing the rate of rotation of a motor 14 to change the peripheral speed of the metering roller 5 and in turn to change the degree of such slip.
FIG. 4 shows a second conventional dampening roller arrangement in the offset printing press, of the type known from JP H03-67630 B, in which the plate cylinder 1 and the form dampening roller 6 are designed to be rotated at different peripheral speeds while slipping at their mutual area of contact S3.
Here, the form dampening roller 6 is driven coupled to a unit drive means for rotationally driving the plate cylinder 1 or any other roller. The water fountain roller 4 in part dipped in the water tank 3 and the form dampening roller 6 are rotated at mutually different peripheral speeds while slipping at their mutual area of contact S4. In this known dampening roller arrangement, too, it is designed that the water fountain roller 4 and the metering roller 5 are geared to each other and the metering roller 5 is rotationally driven by the motor 14.
FIG. 5 shows a third conventional dampening roller arrangement in the offset printing press, of the type known from JP H11-198338 A. Here again, water in the water tank 3 is supplied onto the plate cylinder 1 via the water fountain roller 4, the metering roller 5 and the form dampening roller 6. In this arrangement, however, the form dampening roller 6 is coupled to the plate cylinder 1 via a coupling device 16 having a speed selector 15 which permits a choice to be made to adopt either the slip free drive path in which the form dampening roller 6 and the plate cylinder 1 are rotated at an identical peripheral speed or the slip drive path in which the form dampening roller 6 is rotated at a peripheral speed slower than that of the plate cylinder 1, thereby slipping relative to the plate cylinder 1.
In the first mentioned type of prior arrangement in which the form dampening roller 6 and the plate cylinder 1 are rotated at an identical peripheral speed without slipping, the absence of wiping actions by slipping in the mutual area of contact S1 between the plate cylinder 1 and the form dampening roller 6 tend to cause paper powder, dust, dried ink membrane or the like to adhere on the plate cylinder 1, thereby giving rise to a “hickey” being imperfections in printing areas and spots in nonprinting areas in a printed sheet of paper.
In the second mentioned type of prior arrangement in which the form dampening roller 6 is rotated by the unit drive means at a peripheral speed differing from that of the plate cylinder 1, slipping in the mutual area of contact S3 enables obviating the hickey. In this prior arrangement, however, continuous slipping contact between the form damping roller 6 and the plate cylinder 1 with the slip ratio held high results in the form dampening roller 6 rubbing the plate cylinder 1, and hence tends to shorten the service life of the form plate.
The third mentioned type of prior arrangement permits the slip ratio in the mutual area of contact between the plate cylinder and the form dampening cylinder 6 to be switched between two stages. The slip ratio in each stage is held constant, however. The problem is thus encountered with this prior arrangement that it does not tend itself to optimizing the slip ratio for each of a variety of different jobs.